The phrase abdominal pain-related functional gastrointestinal (GI) disorders (APFGIDs) has replaced the term recurrent abdominal pain to describe a condition affecting 10%-46% of school age children worldwide. It exerts a tremendous economic, social, and emotional burden and in up to 60% of children progresses into adulthood. Management and treatment are hampered by lack of biomarkers to characterize pathophysiologically what is a phenotypically and arbitrarily defined condition. We propose to study novel GI and serum biomarkers and use measures of psychosocial distress our preliminary data suggest likely characterize a substantial subset of children with APFGIDs. Our study also will provide an innovative and significant new opportunity to understand the pathobiology of APFGIDs. This knowledge likely will lead to more effective management and treatment strategies. Thus, building on our previous work, we propose the following SPECIFIC AIMS: 1) Compare GI permeability and microbiome composition (GI biomarkers) in children (7-12 yr. of age) with APFGIDs (n=150) vs. Healthy Children (HC) (n=75) without abdominal pain. Hypotheses - Children with APFGIDs vs. HC have: H1) Increased GI permeability and H2) A GI microbiome enriched with Gammaproteobacteria and Alistipes. 2) Among children with APFGIDs, compare abdominal pain symptoms (frequency/severity) and psychosocial distress in those with abnormal vs. normal GI biomarkers. Hypotheses - Among APFGID children, those with: H3) Increased GI permeability or H4) A GI microbiome enriched with Gammaproteobacteria/Alistipes will have greater abdominal pain symptoms but less psychosocial distress compared to those with normal permeability or microbiome composition. 3) In children who agree to have blood drawn (70-80% of the sample from Aim 1), compare serum immune markers (lymphocytes and cytokines) in children with APFGIDs versus HC. Sub-aim: explore how changes in serum lymphocytes and cytokine profiles relate to abdominal pain symptoms in children with APFGIDs. Hypotheses: H5) APFGID children will have decreased CD19+/increased CD8+ lymphocytes and an increased proportion of serum proinflammatory cytokines vs. HC; H6) In children with APFGIDs, the degree of immune marker alterations will correlate with abdominal pain symptoms. 4) Explore patterns of associations among biomarkers and differentiation of subgroups. Our multidisciplinary approach addresses NIH Pain Consortium and NINR goals to address a vulnerable population (children), who often go on to become adults with chronic abdominal pain with its attendant cost of medical care (57% greater vs. healthy). Our proposed study is important because health care providers working with patients with APFGIDs are challenged given the underlying pathobiology remains poorly defined and treatments are not universally effective. Our results could shift the paradigm of treatment from one-size-fits-all to targeted management (e.g., probiotics for those with abnormal GI markers and low psychosocial distress).